1. Field of the Invention
The present invention relates to a thermal printer which performs printing on recording paper passed between a platen roller and a print head, and which is effective for detection of the presence or absence of the recording paper and detection of a set state of the print head.
2. Description of the Related Art
A thermal printer provided with a thermal head having a heating element and a platen roller is used in a cash register that prints and outputs receipts, a ticket dispenser that dispenses tickets by printing the tickets on recording paper, and the like.
Generally, such a thermal printer performs paper feed while performing printing by pressing the platen roller against the thermal head and rotating the platen roller. Here, there are broadly three kinds of systems for setting recording paper in the thermal printer: a manual feed system where the print head is set separated from the platen roller and the recording paper is manually inserted between the print head and the platen roller; an automatic insertion system where when the recording paper is inserted between the print head and the platen roller, a sensor detects this situation and the platen roller automatically makes rotation and pulls in the recording paper; and a platen attachment/detachment system where the platen roller is detached, the recording paper is loaded, and the platen roller is attached again. Regardless of which one of these systems is adopted, the conventional thermal printer is basically provided with two kinds of sensors, one of which is used to detect the presence or absence of the recording paper and the other of which is used to detect whether the head and the platen roller are set at their predetermined positions.
The reason why the two sensors are provided in this manner is that in the thermal printer, print information is transmitted from a control circuit side to the print head and the head heating element is heated in accordance with the print information, so that if printing is performed under a state where the recording paper does not exist or the platen roller does not contact the head, this results in a situation where print processing is carried out without performing precise printing on the recording paper and the print information is lost.
Also, if the head heating element is continuously driven and heated under a state where the platen roller is separated from the print head, heat is not conducted from the head heating element to the recording paper and stays in the head heating element. Under this state, the head heating element is heated to a high temperature as compared with a state where the print head is pressed against the platen roller, which results in a problem that the head heating element is significantly damaged and its durability is decreased.
Further, in the case where the head heating element is driven and heated under a state where the recording paper does not exist, when the platen roller is pressed against the print head, the platen roller is directly headed. This results in a situation where the platen roller is significantly damaged.
As a conventional technique of solving these problems, there is known a thermal printer provided with a lock detection sensor, such as a mechanical switch, which detects the open/close state of a cover with respect to a printer main body, and a recording paper detection sensor, such as a photointerrupter, which utilizes reflection light, thereby making it possible to detect the presence or absence of the recording paper and the lock/unlock state with reliability when the recording paper is set in the printer and the cover of the printer main body is closed (see Patent Document 1, for instance).
Also, as another conventional technique, there is known a thermal printer where only one photosensor, such as photointerrupter, which utilizes reflection light is provided and, when recording paper is set in the printer but a platen roller is not set at its predetermined position, the photosensor is prevented from being turned on by pushing up the recording paper using a wire spring (see Patent Document 2, for instance).
[Patent Document 1]
JP 2000-46322 A
[Patent Document 2]
JP 2000-46321 A
With the invention disclosed in Patent Document 1 described above, however, the detection of the presence or absence of the recording paper and the detection of the position of the platen roller with respect to the head are performed using the two sensors. Therefore, the number of components is increased, and a control unit that monitors states of these two sensors becomes necessary, which leads to a problem that a control mechanism becomes complicated and increase in costs becomes inevitable. Also, the presence or absence of the recording paper is detected using an optical sensor such as a photosensor, so that there is a possibility that the sensor may malfunction due to scattered light entering from the outside.
Also, with the invention disclosed in Patent Document 2 described above, an urging member, such as a wire spring, which sets the recording paper spaced apart from a paper guide, needs to be provided for the paper guide in addition to the photosensor applied to the recording paper detection, which makes the structure complicated. Also, in order to release the wire spring from a stressed state in an interlocked manner with the opening of the cover and to set the recording paper spaced apart from the paper guide with reliability, it is required to position and dispose the wire spring with high precision, which results in a problem that assembling work becomes cumbersome and a detection mechanism becomes complicated.